1. Field of the Invention
The present invention relates to biocompatible titanium implant for use in dental and orthopedic surgery.
2. Prior Art
Titanium and titanium alloys are lightweight and have superior corrosive resistant properties when exposed to body tissues and body fluids. In addition, titanium and titanium alloys are ideal for use as artificial bones or implant materials (thus called "titanium implant material" in this specification) because they cause less inflammation, hypersensitivity, and allergic reaction when they come in contact with body tissues and body fluids. Titanium and titanium alloys are used for clinical materials for an artificial hip joint, knee joint, osteosynthesis and artificial tooth roots. However, titanium and titanium alloys lack the property to adhere to human bone or coalesce to bone tissue. Therefore, when the titanium and titanium alloy implants are implanted in the body for a long period of time, the metal implant can become detached itself and slip off of the bone.
On the other hand, inorganic materials such as bioactivated glass and hydroxyapatite are known to coalesce with bone tissue in the body. In particular, bioactivated glass is an ideal clinical material needed to bond materials to body tissues. Bioactivated glass has the property to form an apatite layer on its surface in the body or simulated body fluids that contain calcium and phosphoryl ion, and the apatite layer enables bioactivated glass to coalesce to body tissues. However, in many cases, bioactivated glass has lacked sufficient dynamic mechanical properties like toughness. Rather, the titanium and titanium alloys can fulfill these dynamic mechanical properties. Therefore, development of a material has been pursued that has not only the dynamic mechanical properties of metal materials but the bioactivity as of inorganic substance.
For this purpose, titanium implant materials that are spray-coated or sintered with a surface layer of calcium phosphoryl ceramics such as bioactive glass or hydroxyapatite have been proposed. However, the surface layer of the bioactivated material adheres poorly to the titanium implant, and complex production processes are needed to improve adhesion.
Japanese Patent Application Laid-Open No. 1-275766 discloses a solution of such problems and provides an improvement in adhesion of a metallic material to the bioactive layer. The production process in this prior art includes: a step of adding a super fine powder of hydroxyapatite to a hydrolysis solution of alkylsilicate to make a sol, a step of applying the thus obtained sol to the surface of the metal with a dry coating of a titanium alkoxyde solution, and a step of drying the coating of the sol and bake it at a proper temperature. Thus, this process is complicated.
A method for making the surface of a titanium or titanium alloys into a hygroscopic gel has been recognized. This method, however, uses a high concentration alkaline aqueous solution and a heat treatment conducted at a temperature as high as 600.degree. C., making the process very complex. Furthermore, during the high temperature heating process, there exists the possibility that the mechanical properties of titanium and titanium alloy are damaged.
Another method for making the surface of titanium or titanium alloy into a hygroscopic gel is reported by B. Walivaara, I. Lundstrom and P. Tengvall in Clinical Materials (Vol. 12, pages 141-148). However, the thickness of the apatite layer, which is formed on the surface of a test sample after 72 hours of processing, is 12 nm, extremely thin; and a sufficient improvement of affinity to the body is not obtained.